Quantifying the roles of intraspecific and interspecific diversification strategies in forage cropping systems

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Increases in extreme weather events from climate change are likely to hinder forage crop production. Cropping system diversification may be a strategy for improving productivity and enhancing yield stability of forage production systems in the face of climate change, yet farmers are often constrained by their operations as to what strategies they can readily adopt. Moreover, the type of diversity necessary to provide desired outcomes and the potential tradeoffs from such outcomes are largely unknown. We compared three strategies to increase cropping system diversity via intercropping in winter annuals, summer annuals, and perennials in a three-year forage crop experiment conducted in New York, New Hampshire, and Vermont, USA. For each crop type, three diversification strategies, (1) cultivar diversity (Intraspecific), (2) crop species diversity (Interspecific), and (3) both cultivar and species diversities (Intra+Inter), were compared against a control of a single cultivar of a single species (Baseline). Measured responses included yield, weed biomass, forage nutritive value, and yield stability. Across all crop types, yields in Interspecific and Intra+Inter treatments were 15% and 14% greater than the Baseline treatment, respectively. Species diversity had the greatest effect on performance in perennials, which we partially attribute to temporal diversity indicated by differences in biomass composition across harvests within a season. Perennial diversity treatments that contained species diversity had lower weed biomass, lower crude protein, and greater fiber content than the Baseline or Intraspecific treatments. Stability analysis of yields across growing conditions showed that perennial crop yields were consistently greater in the Inter and Intra+Inter treatments than the Baseline treatment. In the summer annuals, yields of the Intra+Inter treatment were greater than yields of the Baseline treatment in sub-optimal environments (20th percentile of the environmental index), whereas, in winter annuals yields in the Intra+Inter treatment were greater than yields of the Baseline treatment in optimal environments (80th percentile of the environmental index). Across crops, strategies of species diversity had a greater impact on responses than cultivar diversity and no additional benefits were detected in strategies that also incorporated cultivar diversity. Benefits of mixtures could be further enhanced by selecting specific traits that maximize both functional and response diversity. Future research should aim to develop guidelines for species and trait compatibility in mixtures and appropriate seeding rates to optimize performance.